The research proposed here addresses internal and external regulators of sexual maturation in vertebrates. Specifically, development of forebrain magnocellular neurons immunoreactive to gonadotropon-releasing hormone (irGnRH) are to be studied in a teleost using cellular and molecular probes. Forebrain irGnRH neurons comprise a phylogenetically ancient system which plays a central role in modulating pituitary control of gonadal maturation in all vertebrate classes. While analysis of this system in mammals is complicated by the superimposition of neocortical functions, basal forebrain organization can be readily studied in teleosts. The irGnRh neurons and their modulation of pituitary function have important consequences to the organism and are often implicated in growth or maturation related disease (hypogonadism and juvenilized physiolgnomy). In the teleost, H. burtoni, both development of irGnRH neurons as measured by some size and complexity of neurite morphology, and gonadal maturation are influenced by social interactions. Competition form larger conspecifics suppresses maturation in young adult fish, causing both undersized irGnRH forebrain neurons and hypogonadism. Since we can control the social system, this system is highly advantageous for analysis of maturation. Two classes of experiments are proposed: One to identify the sensory modality which is responsible for mediating social signals which suppress maturation and the second to ablate two key systems, olfactory bulb and gonads, implicated in forebrain maturation. We will measure the consequences of these experiments with immunoreactivity to GnRH antibody and molecular probes to the mRNA of GnRH. We will thus be able to discover the cellular and molecular consequences of social regulation of maturation. Since the results of social interaction are quite extreme. This may set the stage for understanding a pathway for the social regulation of gene action.